(A) study for improving light extraction efficiency of organic light-emitting diodes

Abstract

In this thesis, two different methods to improve light extraction efficiency of organic light-emitting diode(OLED) - low refractive index array and microlens array - have been studied by using LightTools™ illumination design software, and have also been verified by actual experiments. Since OLED’s first demonstration in 1987 by C.W.Tang and S.A. Vanslyke at Kodak, many academic studies and commercial efforts have been invested in improving OLED device performance. Thanks to all those efforts, it now has established itself a competent position as a candidate for next-generation flat panel display and illumination source. As a next-generation flat panel display, it has many advantages over the rivals such as wide viewing angle, high contrast ratio, thin form factor, etc. However, OLED still has several issues to be resolved before it can be massively produced―issues like device efficiency and lifetime. Many approaches have been introduced to increase the efficiency of OLED devices, which also influences device lifetime. Among the factors that limit OLED device efficiency this thesis focuses on improving the light extraction efficiency of OLED devices. Typical OLED devices that are fabricated on flat glass substrate show ~20% light extraction efficiency. This means that approximately 8 out of 10 photons generated in OLED are wasted while the rays are being waveguided by glass substrate, indium tin oxide(ITO) electrode and organic materials used in OLED First method studied in this thesis is microlens array sheet placed on the backside of the glass substrate. Effect of several parameters like refractive index of the microlens array sheet / lens radius / lens contact angle on OLED light extraction efficiency has been investigated by using LightTools™ software. Actual microlens array sheet was prepared by pouring PDMS on lens-shape patterned photoresist mold and hardening it. Second method is a low refractive index array on top of ITO electrode. Relationsh...